1. Field of the Invention
The present invention relates to repair of turbine blades having an eroded leading edge thereof, and more particularly to a method and apparatus for in situ repair of such blades.
This method is also applicable to initial manufacture of blades since it eliminates expensive silver solder in some cases and restrengthening after brazing.
2. Description of the Prior Art
It is known in the prior art to provide anti-erosion shields for attachment to the leading edges of steam turbine blades used in the final low pressure stages of steam turbines. The purpose of such shields is to reduce the wear on the blades due to cavitation erosion from condensed water particles contained in the steam. A commonly used material for such anti-erosion shields is a Co-W alloy steel known as Stellite.RTM. 6B. Stellite.RTM. is the trade name of a group of nonferrous alloys in which the hardness is an inherent property of the alloy and is not induced by heat treament. In some designs, the anti-erosion shield is attached to a recess in the turbine blade by a joining method welding such as brazing, silver soldering, or electron beam welding or by direct welding of the stellite to the blade leading edge. Other designs use a turbine blade having a hardened leading edge. Although high welding or brazing temperatures may cause distortion during manufacture of the blades, any deformation due to such stresses can be controlled or corrected. For example, some blades are forged after attachment of the shield.
Although anti-erosion shields retard wear of the turbine blades, the first blades in a group often eventually experience preferential wear from water droplets sufficient to seriously reduce the life of these components and the wear leads to reduced turbine efficiency. In the past, it has been necessary to remove the worn blades, weld replacement shield material to the blades, and rework the blades to correct distortions caused by the repair. Such repairs are expensive and time consuming and in some instances, a blade must be replaced.
There is a need for a method of repair in which the anti-erosion shield can be replaced in situ without exposing the blade to high temperatures which could cause distortion. Such a method is also needed for use in initial manufacture to eliminate expensive brazing or welding consumables and distortions due to heat of welding.